Cells of E. amylovora are Gram negative rods, 0.3 x 1-3 µm in size, occur singly, in pairs and sometimes in short chains, and are motile by two to seven peritrichous flagella per cell (see Paulin, 2000, for review).
E. amylovora forms colonies of characteristic colour and colony formation on most culture media (Bereswill et al., 1998). Colonies are domed, circular, mucoid on sucrose nutrient agar (Billing et al., 1961);red to orange on MS medium (Miller and Schroth, 1972);white, circular, mucoid on KB medium (Paulin and Samson, 1973);smooth large, pulvinate, light blue opalescent with craters on CCT medium (Ishimaru and Klos, 1984);and yellow, highly mucoid or less mucoid on MM2Cu media (Bereswill et al., 1998).
Water-soaked flowers, spurs, or shoot tips accompanied by ooze production, followed quickly by necrosis, are early symptoms of fire blight. These symptoms can be detected in an orchard or nursery by experienced observers, but may be overlooked by the inexperienced.
A suitable period for inspection is 3-5 weeks after the blossom period. Look for necrotic leaves and branches, withered blossoms, crooked shoot tips, and ooze. Ooze is more likely to be present in the morning when air humidity is high and host water potential is positive;later in the day when the air is dry, ooze may be shiny and glassy.
Cankers may form on branches and trunks at the junction between infected and healthy bark tissues;therefore, inspections may be needed every 5-7 days throughout the summer or until no new infections are observed.
In autumn, mummified fruits and leaves hanging on dead branches is an indication of fire blight. In winter, the debris helps in locating cankers since the darker bark associated with old infection can blend in with the dormant healthy bark, particularly on older trees.
Related invasive species
- Erwinia amylovora
Related Farm Practice
- Light
- Host plants
- Hosts
- Rootstocks
- Production
- Orchards
The long distance spread of fire blight is a rare event which in most cases seems to be the result of plants or plant tissues being moved across the oceans. Short distance spread is the result of the characteristics of the pathogen, especially its ability to produce an exudate (bacteria embedded in exopolysaccharides) which is easily transported by wind, rain, insects or birds. This is very efficient;once the pathogen has moved into a new territory it almost always colonizes and becomes established. This is accompanied by economic losses in regions where apple, pear or loquat are grown commercially;it might prevent the survival of local cultivars and could disrupt international trade. To date fire blight has colonized most of North America, Western Europe and most of the countries around the Mediterranean Sea as well as New Zealand. Outbreaks of fire blight are irregular and difficult to control.
Fire blight's basic symptom is necrosis or death of tissues. Droplets of ooze on infected tissues are also an important symptom;they are the visible indication of the presence of fire blight bacteria. Except for minor differences, the symptoms of fire blight are basically the same on all host plants.
Infected blossoms initially become water-soaked and of a darker green as the bacteria invade new tissues. Within 5-30 plus days (commonly 5-10 days), the spurs begin to collapse, turning brown to black. Initial symptoms are often coincident with the accumulation of about 57 degree days, base 12.7°C, from the infection date (Steiner, 2000).
Infected shoots turn brown to black from the tip;shoots often bend near the tip to form a so-called 'shepherd-crook' shape. Shoots invaded from their base exhibit necrosis of basal leaves and the stem. Leaves and fruits may be invaded through petioles or stems or infected through wounds, resulting in discoloration followed by collapse of the leaves and fruit. During wet, humid weather, infected leaves and particularly the fruit often exude a milky, sticky liquid, or ooze containing bacteria.
From infected flowers and shoots, the bacteria may invade progressively larger branches, the trunk and even the rootstock. Infected bark on branches, scaffold limbs, trunk and rootstock turns darker than normal. When the outer bark is peeled away, the inner tissues are water-soaked often with reddish streaks when first invaded;later the tissues are dark brown to black. As disease progression slows, lesions become sunken and sometimes cracked at the margins, forming a canker.
Trees with rootstock blight may exhibit liquid bleeding from the crown at or just below the graft union in early summer. Water-soaked, reddish and necrotic tissues are visible when the outer bark is removed. Trees with infected rootstocks often exhibit yellow to red foliage about a month before normal autumn coloration. Rootstocks such as M.26, M.9 and relatives of M.9 often show these symptoms without evidence of infection in the trunk of the scion. Infection of M.7 and a few other rootstocks occurs following infection of suckers arising from the rootstocks;the infected suckers exhibit typical shoot blight symptoms. Many trees with rootstock blight will die in the first year after infection;the remaining rootstock-infected trees often die within 2-3 years.
Any plant tissues invaded by the bacteria can show ooze production on their surface. This exudate is a specific symptom of fire blight. Depending on weather conditions and on the time of the day, ooze may or may not be produced. It is most frequently observed early in the morning when the host water potential is positive. It may appear in different ways: droplets, threads or film on the plant's surface.
E. amylovora is a pathogen of plants in the family Rosaceae;most of the natural hosts are in the subfamily Maloideae (formerly Pomoideae), a few belong in the subfamilies Rosoideae and Amygdaloideae (Momol and Aldwinckle, 2000). Genera in the subfamily Spiraeoideae have been reported as hosts on the basis of artificial inoculation (van der Zwet and Keil, 1979).
Strains of E. amylovora isolated from one host are pathogenic on most other hosts. This was the case for strains isolated from natural infections on Prunus salicina in the USA (Mohan and Thomson, 1996) and on Prunus domestica and Rosa rugosa in southern Germany (Vanneste et al., 2002a). Rubus strains (see Taxonomy and Nomenclature) are host specific;they are pathogenic on brambles but not on apple and pear (Starr et al., 1951;Braun and Hildebrand, 2005). Also, a few Maloideae strains exhibit differential virulence on apple;for example, strain Ea273 was not pathogenic across the same range of apple cultivars and rootstocks as common strain E4001A (Norelli et al., 1984, 1986).
Within each group of susceptible host plants, species or cultivars may be found with a high level of resistance;such plants may show no, or limited, symptoms under natural conditions or even following artificial inoculation (Forsline and Aldwinckle, 2002;Luby et al., 2002). Lists of resistant cultivars are published for important crops (van der Zwet and Keil, 1979;Zeller, 1989;Thomas and Jones, 1992;Berger and Zeller, 1994;van der Zwet and Bell, 1995;Bellenot-Kapusta et al., 2002).
Wild Pyrus (P. amygdaliformis, P. syriaca) in southern Europe and in the Mediterranean area, Crataegus (C. oxyacantha [ C. laevigata ], C. monogyna) in northern and central Europe, and ornamentals (Pyracantha, Cotoneaster, Sorbus) throughout Europe are important sources of inoculum for apple and pear orchards.